This invention relates to apparatus for charging a dielectric material, primarily for use in reproduction systems of the xerographic, or dry copying, type; more particularly, the invention relates to a device for laying down a constant density surface charge on a dielectric having a conductive backing, such as a photoconductive belt, web or drum.
According to the invention of C. F. Carlson as described in U.S. Pat. No. 2,297,691, there is provided a process for preparing electrophotographic pictures wherein a uniform electrostatic charge is applied to the surface of a photoconductive insulating body. This charge is selectively dissipated by exposure to a pattern of light and shadow. This exposure and its consequent dissipation of electric charge results in an electrostatic latent image which can later be developed or made visible by treatment with an electroscopic material which adheres to the electrostatic charge pattern and which, optionally, may be transferred to a second surface to form an electrophotographic print or picture. If desired, other methods of utilization of the electrostatic latent image are available and the basic invention has wide applications in many fields of use.
In electrostatographic reproductive devices it is necessary to charge a suitable photoconductive or reproductive surface with a charging potential prior to the formation thereon of the light image. Various means have been proposed for the application of the electrostatic charge or charge potential to the photoconductive insulating body of Carlson's invention; one method of operation employs, for charging the photoconductive insulating layer, a form of corona discharge wherein an adjacent electrode comprising one or more fine conductive bodies maintained at a high electric potential causes deposition of an electric charge on the adjacent surface of the photoconductive body. Examples of such corona discharge devices are described in U.S. Pat. No. 2,836,725, issued May 27, 1958, to R. G. Vyverberg and U.S. Pat. No. 2,922,883, issued Jan. 26, 1960 to E. C. Giamio, Jr. In practice, one corotron (corona discharge device) may be used to charge the photoconductor before exposure and another corotron used to charge the copy sheet during the toner transfer step. Corotrons are cheap, stable units, but they are sensitive to changes in humidity and the dielectric thickness of the insulator being charged. Thus the surface charge density produced by these devices may not always be constant or uniform.
As an alternative to the corotron charging systems, roller charging systems have been developed. Such systems are exemplified by U.S. Pat. No. 2,912,586, issued Nov. 10, 1959 to R. W. Gundlach; U.S. Pat. No. 3,043,684, issued July 10, 1962 to E. F. Mayer; U.S. Pat. No. 3,398,336, issued Aug. 20, 1968 to R. W. Martel et al. (two phase liquid film interposed between and in contact with dielectric layer and charging roller); U.S. Pat. No. 3,684,364, issued Aug. 15, 1972 to F. W. Schmidlin; and U.S. Pat. No. 3,702,482, issued Nov. 7, 1972 to Dolcimascolo et al.; and Defensive Publication T875,026, published June 30, 1970 in the name of P. T. Scharf. All of these prior art devices are concerned with contact charging, that is the charging roller is placed in contact with the surface to be charged, e.g. the photoreceptor or final support (paper) sheet.
Surface contact charging rollers of the above-mentioned prior art type are restricted to a speed of rotation which is controlled by the speed of movement of the surface to be charged. In other words, because the charging roller contacts the support member, whether it be the photoconductor drum or belt or a paper sheet to which toner is to be transferred, the surface velocity of the charging roller must be equal to the velocity of the chargeable support member. In such systems the roller materials must, in general, be tailored to the particular application and the amount of charge placed on the chargeable support is usually only controlled as a function of the voltage applied to the charging roller.
A principal object of this invention is a device which will lay down a constant density of surface charge on a material passing by the charging station at a uniform speed.
It is a further object of the present invention to combine advantageous features of both the non-contact charging corotron system and the surface contact charging roller system to provide an improved non-contact charging system.
A further object of the invention is to increase the number of parameters available for controlling the application of charge density to the chargeable support. More specifically, it is a feature of this invention to control the charge density applied to the support by either changing the voltage applied to the charging roller or by changing the rotation rate of the charging roller.
A still further feature of this invention is to provide a non-contact charging system which operates somewhat like the wire corotron but which can be continuously cleaned to avoid unwanted accumulations of contaminants that would otherwise affect the charge applied to the support.